Modeling and Simulation of Dfig to Grid Connected Wind Power Generation Using Matlab

8/9/2019 Modeling and Simulation of Dfig to Grid Connected Wind Power Generation Using Matlab

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International Journal of Electrical Engineering and Technology (IJEET), ISSN 0976 – 6545(Print),

ISSN 0976 – 6553(Online) Volume 6, Issue 1, January (2015), pp. 29-40 © IAEME

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MODELING AND SIMULATION OF DFIG TO GRID

CONNECTED WIND POWER GENERATION USING

MATLAB

Mohamed Amer Abomahdi

Research Scholar in the Department of Electrical EngineeringShepherd School of Engineering & TechnologySHIATS Deemed University, Allahabad (India)

Dr. A.K. Bharadwaj

Associate Professor in the Shepherd School in Department of Electrical EngineeringSam Higganbottom Institute of Agriculture

Technology and Sciences - Deemed University,Allahabad (India)

ABSTRACT

The present research work deals with the control of grid frequency by controlling theoperation of doubly fed induction generator and also control regulation of active power of demandand supplied by grid with help of doubly fed induction generator. The evolution of technologyrelated to wind systems industry leaded to the development of a generation of variable speed windturbines that present many advantages compared to the fixed speed wind turbines. These windenergy conversion systems are connected to the grid through Voltage Source Converters (VSC) tomake variable speed operation possible. The studied system here is a variable speed wind generationsystem based on Doubly Fed Induction Generator (DFIG). The rotor side converter (RSC) usually

provides active and reactive power control of the machine while the grid-side converter (GSC) keepsthe voltage of the DC-link constant. The additional freedom of reactive power generation by the GSCis usually not used due to the fact that it is more preferable to do so using the RSC. This paper dealswith the introduction of Doubly fed induction generator, AC/DC/AC converter control and finallythe SIMULINK/MATLAB simulation for isolated Induction generator as well as for grid connectedDoubly Fed Induction Generator and corresponding results and waveforms are displayed.

Keywords: DFIG, Rotor Side Converter, Grid Side Converter, Converter Control Diagram,Simulink Diagram, Wind Turbine Modeling, Wind Energy.

INTERNATIONAL JOURNAL OF ELECTRICAL ENGINEERING &

TECHNOLOGY (IJEET)

ISSN 0976 – 6545(Print)

ISSN 0976 – 6553(Online)

Volume 6, Issue 1, January (2015), pp. 29-40

© IAEME: www.iaeme.com/IJEET.asp

Journal Impact Factor (2014): 6.8310 (Calculated by GISI)www.jifactor.com

IJEET

© I A E M E


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1. INTRODUCTION

Nowadays the demand of electrical energy is increasing day by day but the presence of coal,fossils fuels are towards the end. So it is very much required to find another way to generate theelectricity. Wind energy is a non conventional source of energy and often installed in remote, rural

areas which areas usually have weak grids, often with voltage unbalances and under voltageconditions. Wind energy has been the subject of much recent research and development .Withincreased penetration of wind power into electrical grids, DFIG wind turbines are largely deployeddue to their variable speed feature and hence influencing system dynamics. This has created aninterest in developing suitable models for DFIG to be integrated into power system studies. Thecontinuous trend of having high penetration of wind power, in recent years, has made it necessary tointroduce new practices. For example, grid codes are being revised to ensure that wind turbineswould contribute to the control of voltage and frequency and also to stay connected to the hostnetwork following a disturbance. Renewable energy sources not contributing to the enhancedgreenhouse effect, especially wind power, are becoming an important component of the totalgeneration. Hence, research concerning the dynamic behavior of wind energy systems is important toachieve a better knowledge. In response to the new grid code requirements, several DFIG modelshave been suggested recently, including the full-model which is a 5th order model. These models usequadrature and direct components of rotor voltage in an appropriate reference frame to provide fastregulation of voltage.

2. DOUBLY FED INDUCTION GENERATOR

Wind turbines use a doubly-fed induction generator (DFIG) consisting of a wound rotorinduction generator and an AC/DC/AC IGBT-based PWM converter. The stator winding isconnected directly to the 50 Hz grid while the rotor is fed at variable frequency through theAC/DC/AC converter. The DFIG technology allows extracting maximum energy from the wind forlow wind speeds by optimizing the turbine speed, while minimizing mechanical stresses on the

turbine during gusts of wind. The optimum turbine speed producing maximum mechanical energyfor a given wind speed is proportional to the wind speed. Another advantage of the DFIG technologyis the ability for power electronic converters to generate or absorb reactive power, thus eliminatingthe need for installing capacitor banks as in the case of squirrel-cage induction generator.

Figure 1: DFIG and its power flow


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The stator is directly connected to the AC grid, while the wound rotor is fed from the PowerElectronics Converter via slip rings to allow DIFG to operate at a variety of speeds in response tochanging wind speed. Indeed, the basic concept is to interpose a frequency converter between thevariable frequency induction generator and fixed frequency grid. To achieve full control of gridcurrent, the DC-link voltage must be boosted to a level higher than the amplitude of grid line-to-line

voltage. The slip power can flow in both directions, i.e. to the rotor from the supply and from supplyto the rotor and hence the speed of the machine can be controlled from either rotor- or stator-sideconverter in both super and sub-synchronous speed ranges. As a result, the machine can becontrolled as a generator or a motor in both super and sub-synchronous operating modes realizingfour operating modes.

3. WIND TURBINE MODELING

The first wind turbines were based on a direct grid coupled synchronous generator with pitchcontrolled rotor blades to limit the mechanical power in high wind speeds. Therefore, the firstmodeling efforts were devoted to this wind turbine concept The directly grid coupled synchronousgenerator was followed by a directly grid coupled asynchronous squirrel cage induction generator.To limit the power extracted from the wind at high wind speeds, either pitch control or stall controlcan be applied. Many papers on modeling of a wind turbine with a directly grid coupled squirrel cageinduction generator can be found in the literature, both in combination with pitch control and withstall control of the mechanical power, and Nowadays, a more modern variable speed wind turbinewith a doubly fed induction generator has replaced the conventional constant speed wind turbinewith a directly grid coupled squirrel cage induction generator. As the power developed isproportional to the cube of the wind speed it is obviously important to locate any electricitygenerating turbines in areas of high mean annual wind speed, and the available wind resource is animportant factor in determining where the wind farms are sited . Wind turbine rotor of a given ratingis much larger in size than a hydro-turbine.

Rotor EquationA wind turbine operates by extracting kinetic energy from the wind passing through its rotor.The power developed by a wind turbine is given by:

P=1/2 CP ϑ Vw3 A

WhereP power (W),Cp power coefficient,Vw Wind velocity (m/s),A swept area of rotor disc(m2),ϑ density of air (1.225 kg=m3).

The force extracted on the rotor is proportional to the square of the wind speed and so thewind turbine must be designed to withstand large forces during storms. Most of the modern designsare three-bladed horizontal-axis rotors as this gives a good value of peak Cp together with anaesthetically pleasing design .The power coefficient Cp is a measure of how much of energy in thewind is extracted by the turbine. It varies with the rotor design and the relative speed of the rotor andwind to give a maximum practical value of approximately 0.4. As this needs knowledge ofaerodynamics and the computations are rather complicated, numerical approximations have beendeveloped.


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( ) λ β λ

β λ λ 0068.054.0116

5176.0,

21

+

−−=

−

ieC i

p

1

035.0

08.0

113+

−+

= β β λ λ i

Figure 2: Shows Cp(¸; µ) versus ʎ ¸ characteristics for various values of ʎ. Using the actual values ofthe wind and rotor speed. The maximum value of Cp (cpmax=0.48) is achieved for ¯ = 0± and for¸ =

8:1. This particular value of ¸ is defined as the nominal value (ʎ ¸nom).

Figure 2: Cp vs ʎ characteristics

Performance coefficient Cp as a function of the tip speed ratio¸ with pitch angel ʎ as a parameter.

4. AC/DC/AC CONVERTER

Where type and structure of the model is normally dictated by the particular requirements ofthe analysis, e.g. steady-state, fault studies, etc. This has been a popular approach with regard toDFIG modeling, where simulation of converters has been done based on expected response ofcontrollers rather than actual modeling of Power Electronics devices. In fact, it is assumed that the


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converters are ideal and the DC-link voltage between them is constant. Consequently, depending onthe converter control, a controllable voltage (current) source can be implemented to represent theoperation of the rotor-side of the converter in the model. Physical model, on the other hand, modelsconstituting elements of the system separately and also considers interrelationship among differentelements within the system,

5. CONVERTER CONTROL SYSTEM

The back to back PWM converter has two converters, one is connected to rotor side andanother is connected to grid side. Control by both converters has been discussed here. The rotor-sideconverter is used to control the wind turbine output power and the voltage measured at the gridterminals. The power is controlled in order to follow a pre-defined power-speed characteristic,named tracking characteristic.

Figure 3: Rotor converter control block diagram.

For the rotor-side controller the d-axis of the rotating reference frame used for d-qtransformation is aligned with air-gap flux. The actual electrical output power, measured at the gridterminals of the wind turbine, is added to the total power losses (mechanical and electrical) and is


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compared with the reference power obtained from the tracking characteristic. A Proportional-Integral(PI) regulator is used to reduce the power error to zero. The output of this regulator is the referencerotor current Iqr_ref that must be injected in the rotor by converter C rotor. This is the currentcomponent that produces the electromagnetic torque Tem. The voltage at grid terminals is controlledby the reactive power generated or absorbed by the converter C rotor.

6. GRID SIDE CONVERTER CONTROL SYSTEM

The Grid side converter is used to regulate the voltage of the DC bus capacitor. For the grid-side controller the d-axis of the rotating reference frame used for d-q transformation is aligned withthe positive sequence of grid voltage.

Figure 4: Grid side converter control


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7. SIMULINK DIAGRAM

This is the Simulink diagram for a doubly fed induction generator connected to grid side withwind turbine protection schemes involved for protection from single phase faults and ground faults.The system is connected to a 120 KV, 3 phase source which is connected to a 9MW wind farm (6 of

1.5 MW each) via. Step down transformers, fault protection and pi- transmission line.

Figure 5: MATLAB model for the system

The wind-turbine model is a phasor model that allows transient stability type studies withlong, simulation times. In this demo, the system is observed during 50 s.


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8. SIMULINK MODEL OF ROTOR SIDE AND GRID SIDE CONVERTER CONTROLLER

Figure 6: SIMULINK diagram of rotor side converter control system

9. SIMULINK MODEL OF ROTOR SIDE AND GRID SIDE CONVERTER CONTROLLER

Figure 7: SIMULINK diagram of grid side converter's controller


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10. SIMULATION RESULTS

Turbine response to a change in wind speed "Wind Speed" step block specifying the windspeed. Initially, wind speed is set at 8 m/s, then at t = 5s, wind speed increases suddenly at 14 m/s.Start simulation and observe the signals on the "Wind Turbine" scope monitoring the wind turbine

voltage, current, generated active and Reactive powers, DC bus voltage and turbine speed.

Figure 8: Grid voltage, current, active and reactive power.


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Figure 9: Wind turbine voltage, current, generated active and Reactive powers, DC bus voltage andturbine speed


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Figure 10: Grid side a voltage, active power, reactive power, plant voltage

11. CONCLUSION

To obtain the best efficiency the DFIG system is used which is connected to grid side and hasbetter control. The rotor side converter (RSC) usually provides active and reactive power control ofthe machine while the grid-side converter (GSC) keeps the voltage of the DC-link constant. Sofinally we simulated grid side and wind turbine side parameters and the corresponding results havebeen displayed. The faults can occur when wind speed decreases to a low value or it has persistentfluctuations. The DFIG is able to provide a considerable contribution to grid voltage support duringshort circuit periods. doubly fed induction generator proved to be more reliable and stable systemwhen connected to grid side with the proper converter control systems.


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REFERENCES

[1] Hans Øverseth Røstøen Tore M. Undeland Terje Gjengedal‟ IEEE paper on doubly fedinduction generator in a wind turbine.

[2] Richard Gagnon, Gilbert Sybille, Serge Bernard, Daniel Paré, Silvano Casoria, Christian

Larose “Modeling and Real-Time Simulation of a Doubly-Fed Induction Generator Drivenby a Wind Turbine” Presented at the International Conference on Power Systems Transients(IPST‟05) in Montreal, Canada on June 19-23, 2005 Paper No. IPST05-162.

[3] W.Leonhard, Control of Electrical Drives, 2nd ed. Berlin, Germany: Springer-Verlag, 1996.[4] S.Doradla,S. Chakrovorty,and K.Hole,”A new slip power recovery scheme with improved

supply power factor” .IEEE, Trans.Fower Electron,vol,PE-3,no.2,pp.200-207.Apr,1988.[5] Ekanayake, J.B, Holdsworth, L, Wu, X., Jenkins, N. Dynamic modelling of Doubly Fed

Induction generator wind turbines. IEEE Transaction on Power Systems, 2003, 2:803-809.[6] Ahmad M.El-Fallah Ismail and A.K.Bharadwaj, “Enhancement of Static & Dynamic

Response of the Three Phase Induction Motor Under the Effect of the External Disturbancesand Noise by using Hybrid Fuzzy-Pid Controller”, International Journal of ElectricalEngineering & Technology (IJEET), Volume 5, Issue 12, 2014, pp. 295 - 309, ISSN Print:0976-6545, ISSN Online: 0976-6553.

[7] Nadiya G. Mohammed, “Application of Crowbar Protection on DFIG-Based Wind TurbineConnected to Grid”, International Journal of Electrical Engineering & Technology (IJEET),Volume 4, Issue 2, 2013, pp. 81 - 92, ISSN Print : 0976-6545, ISSN Online: 0976-6553.

[8] Partha Das and Sushabhan Biswas, “Fault Tolerance and Power Quality Study of DFIGBased Wind Turbine System”, International Journal of Electrical Engineering & Technology(IJEET), Volume 5, Issue 5, 2014, pp. 110 - 120, ISSN Print : 0976-6545, ISSN Online:0976-6553.

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Modeling and Simulation of Dfig to Grid Connected Wind Power Generation Using Matlab

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